1. Field of the Invention
The present invention relates to an image processing apparatus, such as a digital copying machine or a facsimile apparatus, and an image processing method employed in the image processing apparatus. More particularly, the present invention relates to an image processing method which is able to output input data at a desired resolution and an apparatus employing the same.
2. Description of the Related Art
A known image processing apparatus, such as a facsimile apparatus, reads a document or image by using an image sensor, such as a CCD, and compresses the output signal from the image sensor into binary data by binary processing in order to transmit the image data to a communication partner.
FIG. 2 is a block diagram of an example of signal processing as mentioned above. A reading unit 201 for reading a document (carrying characters, images or the like) comprises: a light source for irradiating a document; a CCD or another photoelectric converting device for converting image information representing the document to electric signals; and a conveyer means for conveying the document in the sub-scanning direction. The analog signal output by the reading unit 201 is quantized into a digital signal by an A/D converter 202. The multi-value digital signal (plural bits per pixel) from the A/D converter is compressed into binary (black and white) data by a binary processing unit 203.
A normal-type facsimile apparatus provides three reading resolutions with respect to the sub-scanning direction: 3.85 lines/mm (standard (STD) mode); 7.7 lines/mm (fine (FINE) mode); and 15.4 lines/mm (super-fine (SF) mode), which can be selectively used.
A user can select a suitable reading resolution by operating an operating unit 204.
Reading of a document by the CCD is synchronized with an XSH signal which is an interrupt signal issued by a CPU 205. Let it be assumed that an XSH signal is issued every two milliseconds. Then, image data of one scanning line can be read in with the resolution of 15.4 lines/mm in two milliseconds. Since one line of FINE resolution mode (FINE-mode line) corresponds to two lines of SF resolution mode (SF-mode lines), image data for one FINE-mode line is obtained for every two XSH signals. Similarly, image data for one STD-mode line is obtained for every four XSH signals. FIG. 3 indicates the relation between XSH signals and data carried by CCD signals during STD mode, showing that four lines of image data are read in to obtain image data for one STD-mode line. As shown in FIG. 3, an XLST signal is issued for every four XSH signals, and one line of image data immediately after an XLST signal has been issued is recognized as valid data, which then is binarized. The other three lines of image data are treated as invalid.
In the FINE mode, image data for one FINE-mode line is obtained for every two lines of image data which are read in, that is, an XLST signal is issued for every two XSH signals. The line of image data immediately after an XLST is valid, and the other line of image data is treated as invalid.
Problems of the above known art will be described with reference to FIG. 4. In the STD mode, as indicated by the STD-mode pixel size of 8 dots/mm.times.3.85 lines/mm (a unit pixel in the main scanning direction.times.a unit pixel in the sub-scanning direction), only data of one line out of every four lines that are scanned is validated, and the data of the other three lines is invalidated. If the invalidated data contains a piece of effective information, thus, the validated data lacks it, and so this processing causes a reduction of image quality. Similarly, during operation in the FINE mode, if invalidated data, which occurs every other line, contains a piece of effective information, the image quality will accordingly deteriorate.